Apparatus utilizing detonation waves for spraying powders



1956 e. P. HAWLEY ET AL 2,774,625

APPARATUS UTILIZING DETONATION WAVES FOR SPRAYING POWDERS Filed 001;. 21, I954 Unitcd States Patent APPARATUS UTILIZING DETONATION WAVES FOR SPRAYING POWDERS George P. Hawley and Edward T. Tune, Speedway, Ind.,

assignors to Union Carbide and Carbon Corporation,

The present invention relates to improved apparatus utilizing detonation waves for spraying powders to coat surfaces and the like. By the term detonation is meant a very rapid combustion in which the flame front moves at velocities higher than the velocity of sound in the unburned gases, and therefore characterized as supersonic velocities. (Typical calculated velocities of sound at normal pressure are 1085 feet per second at 18 C. in a 50% oxygen-50% acetylene mixture, 1384 in the same mixture at 200 C. and 1122 at 18 C. in a 9.5% acetylene-90.5% air mixture; in air at 18 C. the sonic velocity is calculated as 1122 feet per second.) The rate of flame propagation is far greater in a detonation than in an explosion, which is a combustion in which the velocity of flame propagation does not exceed the velocity of sound in the unburned gases. According to Wilhelm Josts Explosion and Combustion Processes in Gases, McGraw-Hill Book Co., Inc. New York (1946), pages 160 to 210 of which are devoted to detonations, the velocity of the flame front in detonations thus far investigated is f-rom l to 4 kilometers per second (about 3,280 to 13,120 feet per second), as compared to, for instance, 50 feet per second for a typical explosion.

The flame of a detonation moves into the unburned gas with a velocity which is supersonic instead of subsonic, and it is initiated by and remains associated with a shock front. Once established in a long tube, the detonation wave travels at a constant velocity (Lewis and Von Elbe, Combustion, Flames and Explosions, Academic Press Inc., 1951).

Heretofore, apparatus has been proposed for utilizing detonation waves and associated phenomena to impart energy to powders provided in a detonatable fluid body and propel said powders at high temperatures and velocities against the surface of a body to be coated. Such apparatus, as disclosed and claimed in application Serial No. 275,332, file-d March 7, 1952, by R. M. Poorman et al., issued on August 2, 1955, as U. S. Patent No. 2,714,563, is termed a detonation gun and comprises an elongated barrel having an open end, means for introducing a detonatable body of detonatable fluid into said barrel, means for providing powder material in said detonatable body of detonatable fluid in said barrel, means for controlling the supply of fluid fuel and V 2,774,625 Patented Dec. 18, 1956 simultaneously open to admit combustible gas and oxidant (with or without the powder) to the elongated barrel of the gun,

2. The poppet valves are then closed,

3. Immediately thereafter, a third poppet valve opens to admit an inert nitrogen gas stream which flows across the combustion gas and oxidant valves to dilute any leaks from such valves which might cause flashback upon detonation of the mixture,

4. Immediately-after the nitrogen valve opens, and while it remains open, spark ignition fires the gun causing a detonation,

5. After detonation occurs, nitrogen from the open valve flows through the gun to drive out the hot combustion products and form a protective Wall between them and the next combustible mixture charge.

6. The nitrogen valve then closes and the cycle is ready to be repeated by the simultaneous reopening of the combustible gas and oxidant valves to form the next detonation mixture.

This timed sequence of valve operation and spark plug firing is accomplished by driving the valves and spark energization circuit from a common cam shaft.

The frequency of the detonations is a factor in attaining effective operation of the detonation gun. The most useful frequency depends on the particular use of the gun, the design of the gun, and the character of the detonating gas mixture. A single detonation suflices when a thin deposit on a small area is desired, for example a tungsten carbide coating 0.0005 inch thick on a steel surface one inch or less in diameter. For making thicker coatings, and coating larger areas quickly severaldetonations per second are usually desirable.

It was found that under some operating conditions, for instance when using oxygen and acetylene as the oxidant and fuel respectively, it is desirable to have positive closure means between the ignition chamber and the gas supplies of the detonation gun. In addition, it is advantageous under some circumstances to introduce the powder downstream of the ignition zone so that powder will not be deposited on the walls of the barrel in that zone. These features are shown and described in U. S. Patent No. 2,714,563.

It has been found that, although it is advantageous to introduce the powder into the detonation gun, through the side of the barrel downstream of the ignition zone, a number of detrimental effects resulted which effect the continued operation of the detonation gun and the quality of the resultant coating.

The primary difliculty encountered in introducing powders into the barrel of the detonation gun in this manner is that excessive powder deposition occurs in the bore of the powder entry tube in the region where it joins the barrel of the detonation gun. Such powder deposition is to be avoided since it reduces the supply of powder to the barrel of the detonation gun. Additionally, these deposits periodically flake off and pass through the barrel of the detonation gun to the workpiece, thereby causing defects in the resultant coating.

It has also been found that the introduction of powder through an entry tube positioned in the side of the barrel of the detonation gun downstream of the ignition zone tends to unequally distribute the powder in the detonatable fluid in the barrel of the detonation gun, thereby resulting in a non-uniform distribution of powder deposition on the internal walls of the barrelof the gun.

Accordingly, it is an object of the invention to provide a detonation gun containing improved means for introducing powder into the barrel downstream of the ignition zone, whereby the deposition of powders will be substantially eliminated.

Another object is to provide such apparatus wherein the buildup of powder on the internal walls of the barrel is redistributed to a more uniform and less harmful pattern.

Other aims and advantages of the invention will be apparent from the following description and appended claims.

In the drawing:

Fig. l is a schematic view of a detonation gun for coating with powders employing the improved powder entry means of the invention, portions having been broken away to show internal construction;

Fig. 2 is an enlarged view of the back portion of the powder entry means of the detonation gun of Fig 1 down to a transverse break line a-a, parts having been broken away to show internal construction; and

Fig. 2a is the forward portion of the powder entry means extending from the transverse break line aa.

In accordance with the present invention, detonation gun D is provided comprising an elongated barrel 10 having an open end 12 from which the products of the detonation and powders pass to the object to be coated. Cooling water circulating jacket 14 is provided around barrel 10 to prevent overheating. Cooling water is supplied to the annular space 16 between jacket 14 and barrel 10 through inlet means 18, and water passes from annular space 16 through outlet means 20.

Fuel and oxidant are supplied to port means 22 and 24, respectively, of mixing block 26 which is provided with poppet valves 28 for regulating the flow of such fluids at timed intervals. Mixing of the fuel and oxidant takes place in mixing chamber 30 of mixing block 26 and the detonatable mixture is passed to barrel 10 of detonation gun D. .Mixing block 26 is also provided with purging gas inlet port means 36 and poppet valve means 38 for purging mixing chamber 30 and barrel 10 of gases in the intervals between each successive detonation and the introduction of the following detonatable charge into elongated barrel 10 of the detonation gun.

Means is provided in the walls of mixing chamber 36 for insertion of ignition means 42, such as a spark plug or the like, into the mixing chamber; and energizing lines 44 are also provided.

Powder entry tube assembly 46 passes through the rear or breech end 48 of cooling water jacket 14 and the elongated barrel 10 and passes axially through the barrel to a point downstream of the zone (in the region of ignition means 42) where ignition and the initiation of the detonation phenomenon occurs. The powder entry tube assembly 46 preferably comprises powder entry tube 50, two concentric cooling fluid sleeves 52 and 54 surrounding entry tube 50 over at least that portion of entry tube 50 which is positioned in the interior of barrel 10, cooling fluid inlet and outlet parts 56 and 58, respectively, and guide bracket means 60 for properly supporting entry tube '0 to prevent bending or breakage.

A suitable powder dispenser 61 is employed for suspending the material to be coated in a gas stream in which it is carried into the barrel of the detonation gun. It has been found that the length of line 62 between the powder dispenser and the discharge end of the powder entry tube 54 should be sufiiciently great to prevent the back surging of gases resulting from the detonation in the gun barrel and consequent disruption of the supply of powder to the gun barrel. For example, in order to maintain continuous non-erractic operation, the minimum length required for a powder tube of /s inch inside diameter is 8 feet 4 inches.

Cooling fluid passes into powder tube assembly 46 through inlet 56 to an annular space 63 between inner cooling fluid sleeve 52 and powder entry tube 50 down the length of the powder tube and returns to outlet 58 through annular space 64 between inner cooling fluid sleeve 52 and outer cooling fluid sleeve 54. In this manper cooling fluid is continually circulated through and around the cooling passages over the entire length of that portion of the powder tube which is positioned in the internally hot region within the interior of barrel 10.

The powder entry assembly 46 shown in Figs. 2 and 2a of the drawing is mounted in such a way as to provide for easy removal and replacement of powder entry tube 50 in the assembly. The procedure for removal and replacement is as follows: The entire assembly is first removed from the detonation gun by loosening coupling 66 at the breech end 48 of the gun. Upon removal of the assembly from the gun, the nose nut 68 is removed and retaining nuts 75 and 76 are loosened. Thereupon powder entry tube can be removed and a new tube 50 may be then inserted in the assembly by reversing these steps.

it has been found that axial powder entry tubes in accordance with the invention have an inherent life of about five hours total coating time at the end of which the abrasive action of the powder has severely damaged the tube to the extent that replacement is required. Accordingly, it is of great importance that a powder tube assembly is provided which permits rapid removal and replacement of such axial powder entry tubes in a minimum time.

It has also been found that the diameter of the powder entry tube can be of any convenient size, the maximum value being determined by cooling fluid requirements (larger diameter tubes would require cooling jackets which would nearly fill the interior of the gun barrel) and the minimum diameter limitation is determined by the powder feed rate required for coating.

The use of an axial powder entry tube in accordance with the invention accomplishes the following: It completely stops powder deposition in the powder tube bore and the accompanying decrease or stoppage of powder supply to the gun barrel; it redistributes the buildup of powder deposit within the gun barrel to a more uniform and, therefore, less harmful pattern; it greatly decreases the number of porous powder agglomerates deposited on the surface to be coated, thereby reducing the number of defects in the coating; and it decreases the maintenance time required on the powder entry tube.

Satisfactory coatings can be obtained employing a detonation gun having axial entry tube means in accordance with the invention where the powder entry tube is inserted in the barrel to any distance from the breech up to a point downstream of the ignition zone. It has been found preferable, however, that the powder entry tube should be inserted at least to a point downstream of the ignition zone to reduce the powder deposition in the barrel and obtain high quality coating.

It has been discovered that it is important to insure axial positioning of the powder entry tube, and cooling of the tube has been found necessary due to the extremely high temperatures attained within the barrel. Non-cooled powder entry tubes have been employed for limited times but in no instance was it possible to operate for longer than one minute periods without pro-ignition of the detonatable mixture and a consequent failure in obtaining the required detonation.

What is claimed is:

1. In a detonation gun having an elongated barrel having an open end, said barrel having a length-to-diameter ratio suflicienitly high to permit the formation of a detonation therein, mixing chamber means directly and continuously communicating with said barrel for forming and passing to said barrel charges of detonatable fluid fuel mixture, means for supplying the components of said detonatable fluid fuel mixture to said mixing chamber means, supply means associated with said barre-l for providing comminuted solid material in said detontable fluid fuel mixture, and means associated with said barrel for igniting said fluid fuel mixture in said barrel to initiate said detonation and propel said comminuted solid material from said gun, the improvement which comprises employing, as said supply means, powder entry tube means passing substantially axially through said elongated barrel to a point downstream of said igniting means for introducing a gas-borne stream of powder into said fluid fuel mixture in said barrel.

2. Apparatus in accordance with claim 1, also containing means for circulating a cooling fluid through the interior of said elongated barrel of said gun and in heat exchange contact with said powder entry tu-be means.

References Cited in the file of this patent UNITED STATES PATENTS Hansen May 1, 1945 FOREIGN PATENTS Great Britain of 1943 

